1. Field of Invention
The present invention relates in general to purification of an exhaust emission from an internal combustion engine, and more particularly to an apparatus and a method which permit efficient recovery of an original exhaust-emission purifying ability of a catalyst used for purifying the exhaust emission of the internal combustion engine, after the ability has been lowered due to absorption or adsorption of specific substances contained in the exhaust emission.
2. Description of Related Art
As one example of NOx purifying catalysts whose exhaust-emission purifying ability is lowered due to absorption or adsorption of specific substances contained in the exhaust emission, there is known an NOx occlusion-reduction catalyst which is capable of selectively occluding and holding nitrogen oxides (NOx) contained in the exhaust emission, by adsorption and/or absorption of the NOx, when the exhaust emission to which the catalyst is exposed has a fuel-lean air/fuel ratio, and is capable of reducing the occluded NOx, by using a reducing component contained in the exhaust emission, when the exhaust emission has a stoichiometric or fuel-rich air/fuel ratio. Like the NOx, sulfur oxides (SOx) also present in the exhaust emission are occluded, by adsorption or absorption, in the NOx occlusion-reduction catalyst. Unlike the NOx, however, the SOx occluded in the NOx occlusion-reduction catalyst cannot be released from the NOx occlusion-reduction catalyst at a temperature near a temperature of reduction of the NOx, even when the exhaust emission has a stoichiometric or fuel-rich air/fuel ratio. Accordingly, the amount of the SOx occluded in the NOx occlusion-reduction catalyst gradually increases with an increase in the cumulative operating time of the internal combustion engine.
An increase in the amount of the SOx occluded in and held by the NOx occlusion-reduction catalyst reduces the volume of a portion of the catalyst in which the NOx can be occluded. In other words, the increase in the amount of the SOx occluded in and held by the NOx occlusion-reduction catalyst lowers the NOx occluding (purifying) ability of the NOx occlusion-reduction catalyst, causing an increase in the amount of NOx that is not occluded in the catalyst and merely passed through the catalyst, namely, an increase in the amount of NOx that is left in the exhaust emission to be discharged into the atmosphere, without purification by the catalyst. In the present specification, a phenomenon of reduction of the NOx occluding ability of the catalyst due to occlusion of the SOx, that is, a phenomenon of reduction of the NOx purifying ability of the NOx occlusion-reduction catalyst will be referred to as “SOx poisoning” of the NOx occlusion-reduction catalyst. Namely, the NOx occlusion-reduction catalyst is poisoned by adsorption or absorption of specific substances in the form of sulfur oxides contained in the exhaust emission, so that its exhaust-emission purifying ability is lowered.
The NOx purifying ability of the NOx occlusion-reduction catalyst which has been lowered due to occlusion of SOx can be increased back to its original ability, by releasing the occluded SOx from the NOx occlusion-reduction catalyst. To release the occluded SOx from the NOx occlusion-reduction catalyst, however, the catalyst should be treated to remove the SOx poisoning under conditions different from those used for usual reduction of NOx by the NOx occlusion-reduction catalyst. That is, the temperature of the exhaust emission must be raised to a level higher than the temperature of the usual NOx reduction, as well as controlling the air/fuel ratio of the exhaust emission to be stoichiometric or slightly fuel-rich, as in the usual NOx reduction.
JP-A-6-66129 discloses an example of an exhaust-emission purifying apparatus for an internal combustion engine, which is arranged to effect such a treatment of the NOx occlusion-reduction catalyst so as to remove the SOx poisoning.
The exhaust-emission purifying apparatus disclosed in the above-identified publication uses an NOx occlusion-reduction catalyst which is disposed in an exhaust passage of the internal combustion engine and which is capable of selectively occluding and holding NOx contained in the exhaust emission, by adsorption and/or absorption of the NOx, when the incident exhaust emission has a fuel-lean air/fuel ratio, and is capable of reducing the occluded NOx, by using a reducing component contained in the exhaust emission, when the incident exhaust emission has a stoichiometric or fuel-rich air/fuel ratio. The apparatus is arranged to estimate the amount of sulfur oxides (SOx) absorbed in the NOx occlusion-reduction catalyst, and initiate the treatment of the NOx occlusion-reduction catalyst to remove the SOx poisoning, when the estimated amount of SOx absorbed in the catalyst has increased to a predetermined upper limit. This SOx-poisoning removing treatment is effected by activating an electric heater provided on the NOX occlusion-reduction catalyst to raise the temperature of the catalyst, and changing the air/fuel ratio of an air-fuel mixture supplied to the engine, to the stoichiometric value, so that the NOx occlusion-reduction catalyst is exposed to the exhaust emission having the stoichiometric air/fuel ratio. According to this arrangement, the exhaust emission to which the NOx occlusion-reduction catalyst is exposed has a higher temperature and a lower air/fuel ratio, than in the normal operation of the exhaust-emission purifying apparatus, so that the sulfur oxides (SOx) occluded in the NOx occlusion-reduction catalyst are released from the catalyst, whereby the original NOx purifying ability of the NOx occlusion-reduction catalyst is recovered.
However, the apparatus disclosed in the above-indicated publication JP-A-6-66129 has a drawback in that the SOx-poisoning removing treatment of the catalyst cannot be efficiently effected.
The NOx occlusion-reduction catalyst with a certain amount of SOx occluded therein may or may not perform its function without a problem, depending upon the condition of the catalyst, for example, the operating condition of the engine.
For instance, the rate (mg/sec) of emission of NOx from an engine increases with an increase in the load acting on the engine. Therefore, when the engine is operated with a comparatively high load acting thereon, the NOx occlusion-reduction catalyst is not able to remove the entire amount of NOx contained in the exhaust emission, causing an increase in the amount of NOx that is not occluded in the catalyst and passed through the catalyst, even where the amount of reduction of the NOx occluding ability of the catalyst is comparatively small, i.e., a comparatively small amount of SOx is occluded in the catalyst.
When the engine is operated under a comparatively low load, the amount of emission of the NOx from the engine is relatively small. In this case, therefore, the NOx occlusion-reduction catalyst may be sufficiently able to remove the entire amount of NOx contained in the exhaust emission, even where the amount of SOx occluded in the catalyst would prevent the catalyst from having a sufficiently high NOx purifying ability if the engine is operated under a comparatively high load.
Thus, the NOx occlusion-reduction catalyst having a certain amount of SOx occluded therein may suffer from an insufficient NOx purifying ability in an operation of the engine under a comparatively high load, but have a sufficiently high NOx purifying ability in an operation of the engine under a comparatively low load.
The apparatus disclosed in the above-indicated publication JP-A-6-66129 is arranged such that the SOx-poisoning removing treatment of the NOx occlusion-reduction catalyst is initiated when the amount of SOx occluded in the catalyst has reached a predetermined limit which is held constant irrespective of the operating condition of the engine. For the reason described above, this arrangement does not permit sufficiently high efficiency of the SOx-poisoning removing treatment.
If the limit of the NOx purifying ability of the NOx occlusion-reduction catalyst at which the SOx-poisoning removing treatment of the catalyst is initiated is set to be a comparatively small value suitable for a high-load operation of the engine, the SOx-poisoning removing treatment is unnecessarily initiated even when the catalyst actually has a sufficiently high NOx purifying ability for the engine operated under a comparatively low load for a relatively long time. This results in a problem of an increased amount of fuel consumption by the engine.
If the limit of the NOx purifying ability of the catalyst at which the SOx-poisoning removing treatment is initiated is set to be a comparatively large value suitable for a low-load operation of the engine, the SOx-poisoning removing treatment is not initiated even after the NOx purifying ability of the NOx occlusion-reduction catalyst has actually become insufficient for the engine operated under a comparatively high load. This results in a continuing operation of the engine with a deteriorated state of the exhaust emission.